Conventionally, a latent image formed electrically or magnetically in an electrophotographic image forming apparatus is visualized with an electrophotographic toner (may referred to as a “toner for developing an electrostatic image” or merely as a “toner” hereinafter). In the electrophotography, for example, an electrostatic image (a latent image) is formed on a photoconductor, followed by developing the latent image with a toner, to thereby form a toner image. The toner image is generally transferred to a transfer medium, such as paper, followed by fixed on the transfer medium, such as paper. In the process of fixing the toner image on the transfer paper, a thermal fixing system, such as a heat roller fixing system, and a heat belt fixing system, is widely used because of its energy efficiency.
Recently, in the market, there is an increasing need for increased printing speed and energy saving of image forming apparatuses. To this end, desired is a toner having excellent low temperature fixing ability, and capable of providing high quality images. To achieve low temperature fixing ability of a toner, a softening point of a binder resin used in the toner needs to be set low. When the softening point of the binder resin is low, however, part of a toner image tends to be deposited on a surface of a fixing member during fixing, which will then be transferred to a photocopy sheet, which is so-called offset (may be referred to as “hot offset” hereinafter). Moreover, blocking, which is a phenomenon that heat resistant storage stability of a toner reduces, and thus toner particles are fused to each other especially in a high temperature environment, tends to occur. In addition, there is a problem that a toner is fused on an internal area of a developing unit or a regulating member of the developing unit to pollute inside the developing unit, and a problem that toner filming is caused on a photoconductor.
As a technique to solve these problems, it has been known that a crystalline resin is used as a binder resin of a toner. Specifically, the crystalline resin sharply softens at a melting point of the resin, and therefore a softening point of the toner can be reduced to adjacent to the melting point while securing heat resistant storage stability at temperature equal to or lower than the melting point. Therefore, the low temperature fixing ability and heat resistant storage stability are both achieved.
As a toner using a crystalline resin, for example, disclosed is a toner using, as a binder resin, a crystalline resin obtained through a chain elongation of crystalline polyester with diisocyanate (see PTL 1 and PTL 2). These disclosed toners have excellent low temperature fixing ability, but insufficient hot offset resistance, and therefore do not reach the quality required in the recent market.
Moreover, disclosed is a toner using a crystalline resin having a crosslink structure formed by an unsaturated bond containing a sulfonic acid group (see PTL 3). This toner can improve hot offset resistance compared to toners in the conventional art. Further, disclosed is a technique associated resin particles having excellent low temperature fixing ability and heat resistant storage stability in which a ratio of softening point and melt heat peak temperature, and viscoelastic property are specified (see PTL 4).
These toners using a crystalline resin as a main component of a binder resin have excellent impact resistance due to the properties of the resin, but have weak impression hardness, such as Vickers hardness. Therefore, there are problems that pollution to a regulating member or inside a developing unit is caused due to stirring stress within the developing unit, filming is caused on a, photoconductor, and charging ability or flowability of the toner tends to be impaired due to embedded external additive to toner particles. Moreover, it takes a long time for the toner melted on a fixing medium (transfer medium) during thermal fixing to recrystallize, and therefore hardness of a surface of an image cannot be promptly recovered. As a result, there are problems that variations in glossiness due to a roller mark formed on the surface of the image or damage are caused by a discharge roller in discharging after fixing. Moreover, the hardness is not sufficient even after the hardness of the surface of the image is recovered by recrystallization of the toner, a resulting image does not have sufficient resistance to scratches or abrasion.
Further, disclosed is a technique for improving stress resistance of a toner by specifying duro mater hardness of a crystalline resin, and adding inorganic particles in the toner (see PTL 5).
However, such toner cannot improve damages (image transport damage) of a roller mark just after fixing, and image hardness after recrystallization is also insufficient. Moreover, the inorganic particles significantly adversely affect low temperature fixing ability of the toner, and therefore an advantage of the crystalline resin to the fixing ability cannot be utilized at the maximum level.
Meanwhile, disclosed are various techniques in which a crystalline resin and a non-crystalline resin are used in combination, unlike the aforementioned conventional art using only a crystalline resin as a main component of a binder resin (see, for example, PTL 6 and PTL 7). These toners can compensate the disadvantage of the crystalline resin in terms of hardness with the non-crystalline resin, but there is a problem that an effect of the crystalline resin to low temperature fixing ability cannot be exhibited at the maximum level.